Transforaminal and Selective Spinal Nerve Injection
Overview
Selective spinal nerve injection and transforaminal epidural injection can be performed using similar techniques. Indeed, the distinction between the two techniques is questionable because the fascial sheath surrounding the spinal nerves is contiguous with the dura mater within the epidural space. A solution injected around spinal nerve may well enter the epidural space, regardless of whether the needle tip is advanced through the intervertebral foramen prior to injection. Nonetheless, many practitioners reserve the term “selective spinal nerve injection” for injections that are performed with the needle tip adjacent to the spinal nerve, outside the intervertebral foramen, and the term “transforaminal injection” for injections that are performed with the needle tip within the intervertebral foramen. The rationale for injecting steroids is that they suppress inflammation of the nerve, which is believed to be the basis for radicular pain. The rationale for using a transforaminal route of injection rather than an interlaminar route is that the injectate is delivered directly onto the target nerve. This ensures the medication reaches the target area in maximum concentration at the site of the suspected pathology.
Level of Evidence
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Much of the rationale and scientific evidence for use of the transforaminal technique for administering epidural corticosteroids arises directly by extrapolation from the evidence for use of the interlaminar technique (see detailed discussion in Chapter 5). Use of the interlaminar technique was far more prominent until the last decade or so. This is likely a result of most injections in prior decades being performed by anesthesiologists, who were most familiar with identifying the epidural space using surface landmarks and a “blind” loss-of-resistance technique without the use of fluoroscopic guidance. With the emergence of widespread availability and expertise in fluoroscopic guidance, more precise needle placement directly adjacent to the spinal nerves directly in the area of inflammation became feasible. In recent years, a number of studies comparing the interlaminar and transforaminal routes of injection have been published, but our knowledge regarding the comparative effectiveness of the two approaches remains incomplete.
Available practice guidelines have not made any distinction between the interlaminar and transforaminal routes of injection, citing a dearth of available literature to guide any recommendation. Nonetheless, examination of the available observational studies and small number of randomized trials suggests that transforaminal epidural injections lead to a significant reduction in pain in patients with acute lumbar radicular pain when compared with observation alone or conservative management without injection therapy (evidence obtained from well-designed controlled trials without randomization leading to a strong recommendation for use of this treatment based on low-quality evidence). There is insufficient evidence to determine the efficacy of cervical transforaminal injection.
The available trials demonstrate more rapid resolution of acute radicular pain associated with acute lumbar disc herniation following epidural injection of steroids via the transforaminal route, and it is in this group that the evidence of efficacy is strongest. The use of this treatment for radicular pain associated with acute disc herniations occurring at the cervical and thoracic levels is common and most experts find this to be a reasonable extrapolation from the existing scientific evidence; however, the risks of the transforaminal route of injection at thoracic and cervical levels must be carefully weighed against the benefit (see further discussion below). Use of epidural steroid injections should include a specific discussion of potential complications, particularly with regard to the transforaminal approach. Transforaminal epidural injections should be performed with appropriate image guidance to confirm correct needle position and spread of contrast before injecting particulate steroid. The use of transforaminal epidural injection of steroids to treat acute radicular pain associated with foraminal stenosis or neurogenic claudication associated with stenosis of the central spinal canal has been less well studied but remains common, again as an extrapolation from their usefulness in those with acute disc herniations. The use of transforaminal epidural injection of steroids for the treatment of nonradicular spinal pain lacks scientific validation.
Cervical Transforaminal and Selective Nerve Root Injection
Anatomy
At typical cervical levels, the ventral and dorsal roots of the spinal nerves join in the vertebral canal to form the spinal nerve at the level of the intervertebral foramen. The foramen faces obliquely forward and lateral. Its roof and floor are formed by the pedicles of consecutive vertebrae. Its posterolateral wall is formed largely by the superior articular process of the lower vertebra and, in part, by the inferior articular process of the upper vertebra and the capsule of the zygapophysial joint formed between the two articular processes. The anteromedial wall is formed by the lower end of the upper vertebral body, the uncinate process of the lower vertebra, and the posterolateral corner of the intervertebral disc. Immediately lateral to the external opening of the foramen, the vertebral artery rises in a cephalad direction just anterior to the articular pillars of the zygapophysial joints (Fig. 6-1). The spinal nerve, in its dural sleeve, lies in the lower half of the foramen. The upper half is occupied by epiradicular veins. The ventral ramus of the spinal nerve issues from the intervertebral foramen, passing forward and lateral onto the transverse process. In strict anatomic terms, what has been termed “selective nerve root injection” would be more precisely termed “selective spinal nerve injection,” as the technique is carried out at the level of the spinal nerve, not the more proximal ventral and dorsal nerve roots. However, the terms “selective nerve root injection” and “selective nerve root block” have permeated the published literature. Arterial branches arise from the vertebral arteries to supply the spinal nerves (radicular arteries) or the spinal cord via the anterior and posterior spinal arteries (medullary arteries) (see Fig. 6-1). Medullary and radicular arterial branches may also arise from the deep or ascending cervical arteries and traverse through the entire length of the foramen adjacent to the spinal nerve (see Fig. 6-1), and it is these spinal segmental arteries that are at risk for penetration during cervical transforaminal injection.
Patient Selection
The most common indication for selective nerve root injection is to place corticosteroid adjacent to an inflamed nerve root that is causing radicular symptoms. Nerve root inflammation may stem from an acutely herniated intervertebral disc causing nerve root irritation or other causes of nerve root impingement, such as isolated foraminal stenosis due to spondylitic spurring of the bony margins of the foramen. Selective spinal nerve injection with local anesthetic has also been employed diagnostically to determine which spinal nerve is causing symptoms when pathology exists at multiple vertebral levels. This information can prove helpful in planning surgical intervention.
 Figure 6-1. Axial view of cervical transforaminal injection at the level of C6. The needle has been inserted along the axis of the foramen and is illustrated in final position within the posterior aspect of the foramen. Insertion along this axis avoids the vertebral artery, which lies anterior to the foramen, and the spinal nerve, which lies within the foramen angled anteriorly toward the interscalene groove. Spinal segmental arteries arise from the deep or ascending cervical artery, enter the foramen at variable locations, and often course through the foramen, penetrate the dura, and join the anterior or posterior spinal arteries that supply the spinal cord (inset). An arterial branch that joins the anterior spinal artery is termed a “spinal segmental” or “medullary” artery. Likewise, arterial branches arise variably from the vertebral artery to supply the nerve root itself (in this illustration, a branch to the nerve root or “radicular” artery is shown); similar branches from the vertebral artery often penetrate the dura to join the anterior or posterior spinal artery. There is great anatomic variation in the vascular supply in this region. The anatomic variant illustrated is shown to demonstrate how a needle can be placed within a small artery that provides critical reinforcing blood supply to the spinal cord during cervical transforaminal injection. Injection of particulate steroid directly into one of these vessels can lead to catastrophic spinal cord injury. (Anatomic descriptions are based on cadaveric dissections carried out in our laboratory. Detailed data appear in Hoeft MA, Rathmell JP, Monsey RM, et al. Cervical transforaminal injection and the radicular artery: Variation in anatomical location within the intervertebral foramina. Reg Anesth Pain Med. 2006;3:270-274.) |
Positioning
The patient lies supine, facing directly forward (Fig. 6-2). The C-arm is rotated 45 to 65 degrees lateral oblique until the neural foramina are clearly visualized (Fig. 6-3). The patient may also be asked to rotate the head away from the side of injection. Although this facilitates access to the side of the neck, the neural foramina and bony elements of the cervical spine will no longer be aligned. This may prove confusing to the inexperienced practitioner.
Block Technique
A 22- or 25-gauge, 3.5-inch spinal or blunt-tipped needle can be used in all patients; some manufacturers provide similar needles in shorter lengths, and a 2.5-inch needle is sufficient for all but the most obese patients. To avoid the vertebral artery and the exiting nerve root, the needle is advanced toward the posterior aspect of the intervertebral foramen midway between the superior and the inferior limits of the foramen (see Fig. 6-3). Care is taken to
be sure the needle tip remains superimposed on the bone of the facet column during advancement. The depth when the needle is in final position is frequently just 1 to 2 inch from the skin’s surface. Thus, extreme care must be taken to assure that the needle is not advanced too deeply before the first fluoroscopy image is taken. This poses some difficulty when using the coaxial technique, as the needle will not remain seated in the tissue along the intended axis until it has been advanced sufficiently. When the needle is first placed in the superficial tissues then released by the operator to take the first radiograph, the needle will flop to one side under its own weight. In order to keep the needle on axis for coaxial placement, the needle can be grasped with a small clamp and aimed accurately. The use of a clamp allows the operator to keep the needle in a coaxial orientation and take radiographs without his or her hands in the x-ray field. In this way, the superior articular process of the facet just posterior to the foramen is first contacted, preventing needle advancement through the foramen and into the spinal canal. Once the needle contacts the facet, it is then walked anteriorly into the foramen and advanced no more than an additional 2 to 3 mm. The depth is then assessed by obtaining an image in the posterior-anterior (PA) plane (Fig. 6-4). To avoid direct trauma to the spinal cord and intrathecal injection, the needle should be advanced no further than halfway across the facet column. When the needle is in final position, the stylette is removed and a short length of flexible IV extension tubing is attached. The use of this flexible extension assures that the final needle position is not altered by placing and removing syringes directly to the needle’s hub. One to two milliliters of radiocontrast is then injected under “live” or real-time fluoroscopy to ensure the needle tip lies in close proximity to the nerve root without any intravascular or intrathecal spread (Fig. 6-4D
be sure the needle tip remains superimposed on the bone of the facet column during advancement. The depth when the needle is in final position is frequently just 1 to 2 inch from the skin’s surface. Thus, extreme care must be taken to assure that the needle is not advanced too deeply before the first fluoroscopy image is taken. This poses some difficulty when using the coaxial technique, as the needle will not remain seated in the tissue along the intended axis until it has been advanced sufficiently. When the needle is first placed in the superficial tissues then released by the operator to take the first radiograph, the needle will flop to one side under its own weight. In order to keep the needle on axis for coaxial placement, the needle can be grasped with a small clamp and aimed accurately. The use of a clamp allows the operator to keep the needle in a coaxial orientation and take radiographs without his or her hands in the x-ray field. In this way, the superior articular process of the facet just posterior to the foramen is first contacted, preventing needle advancement through the foramen and into the spinal canal. Once the needle contacts the facet, it is then walked anteriorly into the foramen and advanced no more than an additional 2 to 3 mm. The depth is then assessed by obtaining an image in the posterior-anterior (PA) plane (Fig. 6-4). To avoid direct trauma to the spinal cord and intrathecal injection, the needle should be advanced no further than halfway across the facet column. When the needle is in final position, the stylette is removed and a short length of flexible IV extension tubing is attached. The use of this flexible extension assures that the final needle position is not altered by placing and removing syringes directly to the needle’s hub. One to two milliliters of radiocontrast is then injected under “live” or real-time fluoroscopy to ensure the needle tip lies in close proximity to the nerve root without any intravascular or intrathecal spread (Fig. 6-4D
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